In many situations, it is desirable to accurately measure a local temperature of a working device under stress. The device under stress may not be used typically because it's drain current degrades due to the stress.
Some prior methods of accurately measuring a local temperature of a device under stress involved the use of a metal line formed at a higher level than a gate of the semiconductor device under stress of elevated temperatures or current and resulted in inaccurate measurements due to different temperatures at the different levels. Thickness variation of the layers between the device under stress and a temperature sensor may also make calibration difficult. Calibration would need adjustments for manufactured devices, which is not practical. Diodes and transistors have also been used, but need sophisticated algorithms and well optimized corrections for temperature calculation. Junctions of such diodes and transistors are in a certain depth of the semiconductor material where the temperature could be lower due to a steep temperature gradient. Such junctions are not at an interface of interest, and are likely at a different temperature due to temperature gradients. Some methods sense temperature closer to the interface of interest, but use multiple pads for MOS gates. Such gates may act as an antenna and collect charge during processing, possibly damaging the device and making it non-representative of the process.
While performing a fast wafer level reliability (fWLR) test, i.e., applying stress to a semiconductor device, it is desirable to find an accurate and less expensive way to measure a local temperature proximate the interface between a substrate and a working area or channel of the semiconductor device under stress.